System for fabrication of semiconductor bodies

ABSTRACT

A system and method is provided for forming semiconductor tear-drop shaped bodies having minimal grain boundaries. Semiconductor material is melted in a capillary tube at the top of a tower, and forced under gas pressure through a nozzle. Separate semiconductor bodies are formed. They are passed through a free fall path over which a predetermined temperature gradient controls solidification of the bodies. The resultant bodies are tear-drop semiconductor bodies of near uniform size with minimal grain boundaries.

FIELD OF THE INVENTION

The invention relates to forming small semiconductor bodies of simplegrain structure.

PRIOR ART

In U.S. Pat. No. 4,021,323, assigned to the assignee of the presentinvention, an energy conversion system is disclosed which is comprisedof a sheet of separate photovoltaic cells having tear-drop shapedsemiconductor cores of one conductivity type, and outer diffusion layersof a second conductivity type. With such cells, much of thesemiconductor material is within a diffusion length of the junction andthus within the active portion of the cell.

The efficiency of units employing tear-drop shaped photovoltaic cells issubstantially increased when the semiconductor bodies are formed withminimal grain boundaries. The present invention provides sphere-likesemiconductor bodies in which the number of grain boundaries isminimized.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention a quartz capillarytube having a nozzle at the lower end is mounted with its axis verticalin a tower with a controlled atmosphere. A cylindrical graphitesusceptor encircles the lower portion of the capillary tube to a pointbelow the nozzle. Both the capillary tube and the graphite susceptor areenclosed in a quartz envelope which is mounted with its axis vertical.The upper end of the quartz envelope has a gas conduit connectedthereto, and the lower end of the envelope converges to a collector cupstructure having a gas purge conduit. An induction heater work coilencircles the glass envelope in the area of the graphite susceptor.

A semiconductor charge placed within the capillary tube is heatedthrough electrical power applied to the work coil. An inert gas such ashelium, is applied to the capillary tube and the glass envelope to purgethe system of air. When the semiconductor material is in a molten state,the inert gas pressure is increased on the capillary tube to force thesemiconductor material through the nozzle of the tube. Moltensemiconductor bodies are formed thereby which fall the length of theglass envelope, and solidify before reaching the collector cup.

Control of particle shape and grain structure is achieved by use ofeither of two embodiments. In one embodiment the semiconductor bodiesgathered in the collector cup are passed through a remelt tower torecrystallize the bodies and reduce the number of grain boundaries. Moreparticularly, a quartz feed tube having a nozzle at the lower end ismounted vertically with its axis in the remelt tower. A cylindricalgraphite feed tube encircles the quartz feed tube inside a hollowcylindrical graphite susceptor. The susceptor is supported with its axisvertical in the remelt tower. An inner cylindrical heat shieldencompasses both the graphite susceptor and a lower portion of thegraphite feed tube. An outer cylindrical heat shield encompasses theinner heat shield. A tubular fused quartz envelope encompasses the heatshields. An induction heater work coil preferably of hour glass shapeencircles the quartz envelope in the area of the graphite susceptor.

Semiconductor particles enter into a feed tube and pass through thenozzle of the feed tube for free-fall through the graphite susceptor.Power applied to the work coil develops a temperature gradient along thesusceptor to raise the temperature of the particles as they fall to apoint above the melting point of the semiconductor material. Thetemperature then gradually decreases along the fall path until thetemperature of the body is well below the freezing point. Semiconductorbodies formed in this manner have few grain boundaries and are quitesuitable for a variety of uses such as in solar cell fabrication.

In a second embodiment of the invention, the shot forming tower and theremelt tower are unitary for both forming small semiconductor bodies andcontrolling the free-fall environment such that as they gradually cool,the particles achieve near uniform shape and minimal grain boundaries.

DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asfurther objects and advantages thereof, will best be understood byreference to the following detailed description of an illustrativeembodiment taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a cross-sectional view of a semiconductor particle formingtower in accordance with the invention;

FIG. 2 is a cross-sectional view of selected sections of the tower ofFIG. 1;

FIG. 3 is a cross-sectional view of selected sections of a remelt towerin accordance with the invention;

FIG. 4 is a cross-sectional view of the remelt tower of FIG. 3 takenalong lines 4--4;

FIG. 5 is a temperature gradient graph taken along the length of theremelt tower heater coil of FIG. 3; and

FIG. 6 is a second embodiment of the invention wherein the forming towerof FIG. 1 and the remelt tower of FIG. 3 are combined into a unitarytower structure.

DESCRIPTION OF PREFERRED EMBODIMENTS FIGS. 1 and 2

Referring to FIG. 1, a rectangular tower 10 has four corner posts 11with frame members 12a-12c interconnecting the posts at three levelsabove a base 13. Floor plates 12d, 12e and 12f rest on frame members12a, 12b and 12c, respectively. Plates 12d, 12e and 12f each have acentral circular hole therethrough. A quartz tube 14 extends downwardfrom plate 12d. The lower end of the tube is fused to a larger quartztube 15. The lower end of tube 15 in turn extends into the upper end ofa slightly larger quartz tube 16. The lower end of tube 16 is connectedby means of a union 17 to a quartz collector cup 18.

A first gas feed tube 19 leads through a capillary end seal 20. A secondgas feed tube 21 leads through a conduit 30 into the interior of tube14.

An induction heating coil 22 encircles the lower end of tube 14. Heatingcoil 22 is connected to an induction heater (not shown), preferably of atype manufactured and sold by the Lapel High Frequency Laboratories,Inc. of New York, New York. In one embodiment, the induction heater hada power rating of 71/2 kilowatts and was operated at 450 kilocycles.

Referring now to FIG. 2, capillary end seal 20 closes the upper end of aquartz capillary tube 23. Tube 23 extends through a capillary clamp 24into the interior of tube 14. Clamp 24 is secured to a top cover plate25. Cover plate 25 and a gasket 27 are secured to the top surface offloor plate 12d. The upper end of tube 14 is secured in a top clampplate 28, which in turn is secured to the lower surface of floor plate12d along with a gasket 29. Plates 25 and 28, and gaskets 27 and 29 maybe secured to the top floor plate 12d by well known means such as boltand nut combinations, not shown.

As above described, the feed tube 19 is flow connected to a conduitdepending from the end seal 20. Feed tube 21 is flow connected to aconduit 30 passing through plates 25, 12d and 28 to the interior of tube14. Both tubes 19 and 21 supply an inert gas such as helium atcontrolled pressures.

Capillary tube 23 converges to a nozzle 23a formed at the lower endthereof. A charge 23b of semiconductor material such as silicon is shownresting in the bottom of tube 23. In the preferred embodiment describedherein, the nozzle has an inside diameter of about 5 mils. A cylindricalgraphite susceptor 31 encircles a lower portion of tube 23, extending toa point slightly below nozzle 23a. Susceptor 31 is positioned inside thetube 14 and is heated by inductive coupling from coil 22.

Tube 15 extends downward through a center floor plate 12e and into theinterior of tube 16. Tube 15 is secured to the upper surface of plate12e by a clamp plate 33 and a gasket 34. Tube 16 is secured to the lowersurface of plate 12e by a clamp plate 35 and a gasket 36. Plates 33 and35, and gaskets 34 and 36 are secured to plate 12e by well known andsuitable means.

Tube 16 extends through a clamp plate 37, a gasket 38 and a lower floorplate 12f. Plate 37 and gasket 38 are secured to plate 12f. The lowerend of tube 16 converges to a union 17 which interconnects the tube 16with collector cup 18. A gas purge tube 40 extends laterally from theunion 17.

In operation, the capillary end seal 20 is removed from the end of tube23, and one or more solid semiconductor bodies forming a charge areplaced in the tube. The semiconductor charge is preferably a rod ofsolid silicon doped to provide particles of desired conductivity typeand resistivity. The semiconductor charge rests at the bottom of tube23, and is encircled by the graphite susceptor 31 and the work coil 22.The end seal 20 is replaced, and an inert gas, such as helium, under apressure of a few pounds per square inch is then applied through thefeed tubes 19 and 21 to quartz tubes 23 and 14, respectively. Initialflow of helium purges the system of air. The helium flows downwardthrough both tubes, and exits at the conduit 40 which vents union 17.Electrical power applied to the heating coil 22 heats the graphitesusceptor 31 which causes the charges to be heated. When thesemiconductor charge is fully molten, helium gas again is appliedthrough the feed tube 19 at a pressure of about 10 psig to force themolten material through the nozzle 23a. Flow of helium gas also ismaintained through conduit 21 and tubes 14, 15 and 16 at a rate of about10 cubic feet per hour to provide a controlled atmosphere in tubes14-16. As the semiconductor material is discharged from nozzle 23a, itdissociates into small droplets which fall under the force of gravityalong the length of tubes 15 and 16 to be collected in the collector cup18.

With the nozzle 23a having an inside diameter of about 3-4 mils, thesemiconductor bodies formed in tower 10 have a nominal diameter in therange of about 10 to 15 mils. It has been found that the droplets aregenerally characterized by an orange peel surface and a tail protrudingfrom one end, and that numerous grain boundaries exist.

A remelt operation is then performed to improve the shape and grainstructure as will now be described.

FIGS. 3 and 4

In FIG. 3, a remelt tower 50 includes a frame comprised of verticalposts 51 and cross frames 52a and 52b which support plates 52c and 52d.The tower 50 extends downward below cross frame 52b to a base such asshown in FIGS. 1 and 2. A quartz feed tube 53 is connected to a quartzvial 53a with a short length of flexible plastic tubing 53b. A nozzle53c is at the lower end of feed tube 53. Tube 53 extends through topplate 54a, and a gasket 55a both supported on top plate 52c. Feed tube53 extends axially within a quartz tube 57.

Tube 57 is secured to the top plate 52c by a clamp plate 54b and agasket 55b. Plates 54a and 54b, and gaskets 55a and 55b are secured toplate 52c. Inside tube 57, a graphite tube 58 encircles a lower portionof tube 53 and extends to a point below nozzle 53c. The lower end oftube 58 is flanged and is seated within a recess in the upper end of acylindrical graphite susceptor 59. Susceptor 59 is supported at thelower end thereof by a cylindrical graphite pedestal 60. An innermolybdenum shield 61 encircles a lower portion of the tube 58, thesusceptor 59 and an upper portion of the pedestal 60. The inner shieldconsists of two half cylinders located on opposite sides of susceptor 59with the edges thereof parallel and spaced apart leaving gaps 61a, as isbest seen in FIG. 4. Both halves are supported by an upper annularflange 60a on the outer surface of the pedestal 60. The inner shield 61in turn is encircled by a second molybdenum shield 62 which isintermediate to tube 57 and shield 61. Shield 62 is supported by a lowerannular flange 60b on the outer surface of the pedestal 60. The shield62 also consists of two halves with the gaps therebetween rotated 90degrees from the gaps 61a in the inner shield 61 as shown in FIG. 4.

A graphite centering ring 63 encircles the tube 53, and is supported byvertical members of shields 61 and 62.

The pedestal 60 extends through a lower plate 52d into the interior of alower quartz tube 65. The flange 60b abuts the top surface of plate 52dto support the pedestal 60. The tube 57 is laterally supported at plate52d by a clamp 66 and a gasket 67. Plate 66 and gasket 67 are secured toplate 52d. Tube 65 is laterally supported at plate 52d by a clamp 68 anda gasket 69 contiguous to the under surface of plate 52d. The clamp 68and gasket 69 also are secured to plate 52d. The structure of the tower50 below cross member 52b is similar to that of tower 10 below crossmember 12b, and thus need not be further described.

Below nozzle 53c and above the graphite pedestal 60, an induction heaterwork coil 70 of about 20 turns encircles the tube 57. It has been foundthat with an hour glass shaped coil, the occurrence of electrical arcsbetween the quartz tube 65 and the outer shields 62 is substantiallydecreased.

By way of example, in the embodiment shown in FIGS. 3 and 4, feed tube53 was approximately 8 inches in length and 0.25 inches in diameter. Thegraphite feed tube 58 is approximately 8 in length, and has an innerdiameter of about 1 inch. Further, the graphite susceptor 59 has alength of about 40 inches and an inner diameter of about 2.5 inches. Theshields 61 and 62 have lengths of approximately 50 and 54 inches,respectively, and inner diameters of about 3 and 4 inches, respectively.The quartz tube 57 is approxmately 60 inches in length and 5 inches indiameter. The work coil 70 may be formed of 3/8" copper tubing, and isdriven by a 60 Kw, 450 Kc induction heater (not shown) preferably of atype manufactured and sold by Taylor-Winfield, Warren, Ohio, ModelS-6000LF.

An inert gas, such as helium or argon, is introduced into the towerthrough a fitting (not shown). When all of the air has been purged, aspower is applied to the work coil 70 the graphite susceptor 59 isheated. The semiconductor bodies formed by the shot forming tower 10 ofFIGS. 1 and 2 flow from the vial 53a into feed tube 53. Thesemiconductor bodies fall to the lower portion of tube 53 where theypile up and slowly feed through the exit at the nozzle 53c. The bodiesthen free-fall, passing through susceptor 59. Coil 70 controls thetemperature gradient of susceptor 59 to maintain the temperature abovethe melting point of the semiconductor material over a path length whichwill ensure that each body is fully melted before leaving the hot zonewithin susceptor 59. The semiconductor bodies thus are melted as theytravel the length of coil 70, and are resolidified as they fall througha controlled temperature gradient section at the lower end of susceptor59. The process of melting and resolidifying in a controlled thermalgradient transforms the polycrystalline bodies produced in the processof FIGS. 1 and 2 into tear-drop shaped bodies of near uniform shape inwhich the number of grain boundaries is substantially decreased. An0.010 inch diameter body treated in the tower of FIGS. 3 and 4 typicallyhas five or less grains.

Referring to FIG. 4 and looking down onto the upper face of plate 52d,the work coil 70 encircles and is concentric to quartz tube 57. The tube57 in turn encircles the molybdenum shield segments 62a and the shieldsegments 61 which are concentric to the graphite susceptor 59.

A suitable coil configuration involves approximately 20 turns whereinthe outer diameter of the 3 upper turns is approximately 10 inches,followed by two turns having an outer diameter of approximately 81/2inches and 10 turns having an outer diameter of about 71/2 inches andtwo turns having an outer diameter of approximately 81/2 inches. Thelower 4 turns of the coil have an outer diameter of about 9 inches.

FIG. 5

FIG. 5 illustrates graphically the temperature gradient along the freefall path through the work coil 70 of FIG. 3.

Referring to FIG. 5, the temperature profile is illustrated on a scaleidentified in terms of reference points along the length of coil 70.

Reference point 1, at the upper end of the coil, is approximately 51/2inches below the nozzle 53b.

Reference point 2 is approximately 11/2 inches below the reference point1.

Reference point 3 is approximately 31/2 inches below the reference point2.

Reference point 4 is approximately 31/2 inches below the reference point3 and reference point 5 is about 7 inches below the reference point 4.

Reference point 6 is about 7 inches below the reference point 5, andreference point 7 is about 31/2 inches below reference point 6.

Reference point 8 is approximately 31/2 inches below the reference point7, and reference point 9 is about 2 inches below the reference point 8.Reference point 9 is at the lower end of the coil 70. Reference points1, 2, 3, 8 and 9 are shown at representative levels in FIG. 4.

A horizontal dotted line 80 of FIG. 5 indicates the melting point ofsilicon, i.e., 1410 degrees C. Thus it will be appreciated that thesystem operates to apply heat to the bodies as they traverse thefree-fall path in a controlled way to make more uniform the internalstructure.

In FIG. 3 the turns of work coil 70 are more closely spaced near the toprelative to those near the bottom of the coil in order to control thetemperature profile in the susceptor 59 in the manner indicated by curve81 of FIG. 5. The temperature profile may be measured by inserting atungsten-rhenium thermocouple in place of feed tube 53. By use of a 60Kw induction heater of the type manufactured and sold byTaylor-Winfield, the desired temperature profile 81 was reached with the60 Kw heater set for full output. In preferred form, the temperaturegradient of curve 81 over the portion of negative slope is about 35 to30 degrees per inch.

This profile provides fast heating of the falling bodies. It is alsoshaped to assure that the bodies are cooled slowly as they pass throughthe silicon melting point.

Final adjustment of the operating conditions may be established byexamination of bodies which have passed through the tower. Bodies whichhave been properly remelted and resolidified have a tear drop shape anda very smooth surface. If the bodies are not completely melted, the peaktemperature should be increased. Bodies subjected to temperatures whichare too high will have a rough surface. Small bodies may require lowertemperatures than largsr ones.

Semiconductor materials such as silicon and germanium expand uponcooling. When they cool rapidly, as in the tower of FIGS. 1 and 2, it isbelieved that an outer skin is formed over a molten core. As the coresolidifies, the skin is ruptured and the excess material is expelled toform an attached secondary protrusion. Although the change in shape isof little significance, bodies of this type are usually highlypolycrystalline, sometimes containing as many as 10,000 crystalletes inan 0.010 inch diameter sphere.

It is believed that the beneficial results of the remelting operationperformed in the tower of FIG. 3 occur because the cooling isdirectional. That is, the freezing is initiated at one point at or nearthe surface of the body and progresses until all of the material issolidified. Because the semiconductor material expands upon freezing,material is pushed ahead of the solid-liquid interface, resulting in aparticle with a "raindrop" or "tear drop" shape. "Tear-drop" shapedcrystals frequently are single crystals, usually have less than fivecrystalletes in an 0.010 inch body. It has been found that "tear-drop"shaped crystals make excellent solar cells, with air mass 1 efficienciesgreater than 10%.

Although the embodiments described are for silicon, other semiconductingmaterials may be formed in the same manner. Germanium solidifies atabout 950 degrees C., so the temperatures indicated should be reducedaccordingly. Materials like gallium arsenide may require constantpressurization due to the volatility of arsenic. Such procedures arewell known to those skilled in the art.

FIG. 6

FIG. 6 illustrates a unitary structure combining the function of thetower of FIGS. 1 and 2 and the remelt tower of FIG. 3.

Referring to FIG. 6, a quartz tube 90 is shown. An insulating ring 91,preferably of alumina, is centered in tube 90. Insulating ring 91supports and locates a susceptor of graphite 92, and two heat shields 93and 94, segmented and oriented like shields 61a and 62a, FIG. 4. Aquartz capillary 95, which may be identical to capillary 23 of FIG. 2,is centered in the upper end of susceptor 92.

The nozzle 95a of the capillary extends into an open area 96 ofsusceptor 92. The lower end of the susceptor 92 is supported by meansincluding an upfacing insulating ring not shown but similar to ring 91.

Work coil 97 surrounds tube 90, and extends the full length of thesusceptor 92. The turns at the top are closer together than therebelow,so that the temperature at the top, surrounding the capillary 95, ishigher than that of the bottom end of capillary 95 by approximately 100degrees C. Work coil 97 will be shaped as a combination of coils 22 and70 (FIGS. 1 and 4) and may be driven by a 25 Kw 450 Kc induction heatcontrol 97a. A suitable unit is made by Taylor-Winfield.

In operation, a semiconductor charge 98 is placed in capillary 95.Thereafter, tubes 95 and 90 are purged with inert gas such as helium orargon. Electrical power is then applied to heater coil 97. When thecharge 98 is molten, inert gas at a pressure of 10 psi is applied tocapillary 95. The semiconductor bodies formed at nozzle 95a fall freelythrough region 96 of susceptor 92, and solidify near the lower end tothe susceptor. The solidified bodies continue to fall, and are collectedin a cup similar to cup 18 of FIG. 2.

The temperature gradient in zone 96 is of the character shown by thenegative slope portion of the curve 81 of FIG. 5.

It is to be understood that the embodiments herein described areillustrative of the invention. Other arrangements may be devised bythose skilled in the art without departing from the spirit and scope ofthe invention as defined by the appended claims.

What is claimed is: .[.1. A system for fabricating small semiconductorbodies, which comprises: the top and hour-glass shape therebelow.
 8. Thecombination set forth in claim 7, wherein said free fall path is of alength and the temperature is at levels therealong to slowly solidifysaid bodies. .[.9. The combination set forth in claim 6 said capillarytube has a gas port and the upper end of said tube has a gas conduitleading thereto for selective flow of inert gas into said capillary tubeand said elongated tube..].